Property where performing the same operation multiple times produces the same result as doing it once. Like pressing an elevator button repeatedly - the elevator still only comes once.
Payment APIs use idempotency so if a customer accidentally clicks 'Pay' twice, they're only charged once. The second identical request is safely ignored.
Idempotency is a design property (not a single cloud service). All major clouds support building idempotent operations via API features (idempotency keys), conditional writes, unique constraints, retries with backoff, and workflow orchestration.